1. Field of the Invention
The present invention generally relates to a direct fuel injection engine in which a fuel is directly injected in a combustion chamber and ignited by a spark plug. More specifically, the present invention relates to a direct fuel injection engine that performs stratified combustion and homogeneous combustion by directly injecting a fuel in the combustion chamber.
2. Background Information
One example of a conventional direct fuel injection engine is disclosed in Japanese Laid-Open Patent Publication No. H11-82028. The direct fuel injection engine disclosed in this reference has a concave cavity or a piston bowl formed on the piston crown surface. In addition, this conventional direct fuel combustion engine forms a suitable stratified air-fuel mixture in the cylinder by arranging a fuel injection valve substantially directly above the piston bowl. This arrangement allows the fuel stream to collide against a peripheral side wall of the piston bowl and form a fuel stream circulation flow towards the center portion of the piston bowl to reduce fuel consumption.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved direct fuel injection engine. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
In the conventional direct fuel injection engine described above, a volume of the stratified air-fuel mixture that is formed after colliding with the piston is determined substantially by the shape of the piston cavity or the capacity of the cavity. In other words, in the above mentioned conventional direct fuel injection engine, the volume of the stratified air-fuel mixture formed via the cavity is always constant regardless of the engine load since the capacity of the cavity is constant. Therefore, a range of engine load conditions that allows excellent stratified combustion operation is limited with the conventional direct fuel injection engine. More specifically, if the cavity size and other control parameter is determined to obtain stable combustion and to realize good fuel efficiency and small exhaust gas emissions during one stratified operating region with a certain engine load and a certain engine rotation speed, then good fuel efficiency may not be realized or the so-called recoil will occur in another stratified operating region with a different engine rotation speed and a different engine load.
For example, when the engine rotation speed is fast, the advance of the crank angle becomes faster compared to when the engine rotation speed is slow, and thus, the time allowed to form an air-fuel mixture becomes shorter. Therefore, if an identical fuel injection timing and an identical fuel injection duration (this is basically proportional to the engine load) are used for both when the rotation speed is fast and slow, then the ignition timing occurs before the combustible air-fuel mixture reaches the vicinity of the spark plug when the engine rotation speed is fast. In order to avoid this problem, it is possible to set the fuel injection timing to be more advanced as the engine rotation speed becomes. However, in such case, the fuel stream injected during an earlier part of the injection might not be received in the cavity of the piston. In particular, considering executing a homogeneous combustion in the full load region, in which the fuel is injected during an intake stroke, the fuel stream must be injected with at least equal to or more than a certain injection opening angle. Thus, the fuel injection timing in the stratified combustion state cannot be arranged to be too early.
Moreover, when the cavity is made with a larger opening in order to make it easier to accept the fuel stream, the depth of the cavity is restricted to be less than a certain depth in view of the compression ratio. Accordingly, it becomes difficult to receive the fuel stream due to an insufficient depth.
Thus, since an air-fuel mixture is always formed via the cavity and ignited during stratified combustion in a conventional direct fuel injection engine, it is difficult to obtain stable combustion and good fuel efficiency as well as small exhaust emission under various conditions in which the engine rotation speed fluctuates between fast and slow. Moreover, the engine load also changes from low-load to high-load or vice versa during the stratified combustion. When the engine load is low during the stratified combustion, the stratified air-fuel mixture in the vicinity of the spark plug tends to be lean in the above mentioned conventional direct fuel injection engine because the air-fuel mixture is formed after the fuel stream collides against the cavity. Thus, the combustion stability is worsened thereby causing the fuel efficiency to deteriorate. On the other hand, when the engine load is high during the stratified combustion, the air-fuel mixture in the vicinity of the spark plug tends to become excessively dense with the above mentioned conventional direct fuel injection engine. Thus, smoke and HC is increased.
Accordingly, one object of the present invention is to expand a range of engine operation conditions that allows excellent stratified combustion operation at a low cost by executing two different stratified combustion operations depending on the engine load and/or the engine rotation speed.
In order to achieve the above and other objects, a direct fuel injection engine of the present invention basically comprises a combustion chamber, a piston, a fuel injection valve, a spark plug and a control unit. The piston includes a top surface having a cavity at a substantially center portion of the top surface. The cavity is defined at least by a peripheral wall surface and a bottom surface. The fuel injection valve is positioned at an upper portion of the combustion chamber substantially on a center axis of the piston. The fuel injection valve is configured and arranged to directly inject a fuel stream inside the combustion chamber in a substantially constant hollow circular cone shape during a compression stroke when the direct fuel injection engine is operating in a stratified combustion region. The spark plug is configured and arranged to ignite the fuel. The control unit is configured and arranged to control operations of the fuel injection valve and the spark plug. The control unit is further configured and arranged to ignite a first air-fuel mixture formed directly after the fuel stream is injected from the fuel injection valve and prior to a majority of the fuel stream striking the cavity when the direct fuel injection engine is operating in a low-load stratified combustion region. The control unit is further configured and arranged to ignite a second air-fuel mixture formed after a majority of the fuel stream is guided toward an upper portion of the combustion chamber above the cavity by the bottom surface of the cavity after the fuel stream first hits the peripheral wall surface of the cavity when the direct fuel injection engine is operating in a high-load stratified combustion region.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention.